1. Field of the Invention
The present invention relates to a radio base station and a communication control method.
2. Description of the Related Art
In an LTE (Long Term Evolution) mobile communication system defined in 3GPP, a transmission power of a signal of a physical uplink shared channel (hereinafter, referred to as PUSCH), which is used when a radio terminal transmits data, is calculated based on propagation loss PL in radio communication between a radio base station (eNB) and a radio terminal (UE) and scheduling information set by the radio base station. Specifically, the transmission power is calculated based on Equation 1 below (refer to 3GPP, TS36.213 (Ve10.0.1), “Physical Layer Procedures” December 2010.).
                    [                  Equation          ⁢                                          ⁢          1                ]                                                                                  P            PUSCH                    ⁡                      (            i            )                          =                  min          ⁢                                    {                                                                                                                  P                        CMAX                                            ,                                                                                                                                                          10                        ⁢                                                                              log                            10                                                    ⁡                                                      (                                                                                          M                                PUSCH                                                            ⁡                                                              (                                i                                )                                                                                      )                                                                                              +                                              P                                                  O                          ⁢                                                                                                          ⁢                          _                          ⁢                                                                                                          ⁢                          PUSCH                                                                    +                                              α                        ·                        PL                                            +                                                                        Δ                          TF                                                ⁡                                                  (                          i                          )                                                                    +                                              f                        ⁡                                                  (                          i                          )                                                                                                                                }                        ⁡                          [              dBm              ]                                                          (        1        )            
In Equation 1 above, the PPUSCH(i) denotes the transmission power of the signal of the PUSCH. The i denotes a subframe number. The PCMAX denotes maximum transmission power of the radio terminal. The MPUSCH(i) denotes the number of Resource Blocks (hereinafter, referred to as RBs) assigned to the radio terminal by the radio base station in a subframe i. In addition, the RB denotes a time-frequency unit in which a bandwidth is 180 kHz (corresponds to 12 subcarriers) and a time length is 0.5 ms (corresponds to 7 resource symbols). The Po—PUSCH denotes an initial value of transmission power per one RB unique to the radio terminal. The α denotes a coefficient for controlling a propagation loss compensation rate. The PL denotes radio wave propagation loss between the radio base station and the radio terminal. The propagation loss denotes a parameter indicating the degree of attenuation at a reception side of transmission power from a transmission side. The ΔTF(i) denotes a coefficient determined from MCS (Modulation and Coding Scheme). The f(i) denotes a correction term of transmission power based on TPC (Transmit Power Control) command transmitted from the radio base station to the radio terminal.
In the LTE, the radio base station performs scheduling for assigning radio resources (RB and MCS), which are required for uplink (hereinafter, referred to as “UL”) and downlink (hereinafter, referred to as “DL”) communication, to the radio terminal in advance, and notifies the radio terminal of information (scheduling information), which indicates the scheduled content, using PDCCH (Physical Downlink Control Channel). In addition, the radio base station determines a radio resource according to radio wave propagation situations between the radio base station and the radio terminal.
When performing the scheduling, the radio base station calculates SINR (Signal to Interference and Noise Ratio) using the transmission power of the radio terminal, and average interference power or thermal noise measured by the radio base station. The radio base station acquires radio wave propagation situations based on the calculated SINR. In this case, it is not possible for the radio base station to acquire information, which indicates the transmission power when the radio terminal has actually transmitted the signal of the PUSCH, from the radio terminal. Therefore, the radio base station should estimate (calculate) the transmission power when the radio terminal has actually transmitted the signal of the PUSCH. In this case, the radio base station calculates the transmission power of the signal of the PUSCH for the radio terminal using Equation 1 above, and then estimates the calculated transmission power as the transmission power when the radio terminal has actually transmitted the signal of the PUSCH.
When calculating the transmission power (for example, the transmission power of the PUSCH) for the radio terminal using Equation 1 above, the radio base station calculates the transmission power by applying, as propagation loss (hereinafter, appropriately referred to as PL), PL of the UL. On the other hand, when calculating the transmission power using Equation 1 above, the radio terminal applies PL of the DL as the PL. Therefore, when the PL of UL is different from the PL of the DL, values of the transmission power calculated using Equation 1 are different from each other in the radio base station and the radio terminal. In addition, when the PL of UL is different from the PL of DL, for example, the following situations (A) and (B) occur. These situations are generally expected to occur. (A) A situation where the radio terminal moves at a high speed, so that fading variation rapidly occurs in a radio environment between the radio base station and the radio terminal, resulting in a big difference of fading gain between the UL and the DL. (B) A situation where in an environment in which communication is relayed by an AF (Amplify and Forward) relay node having different amplification rates in the UL and the DL between the radio base station and the radio terminal, signals are transmitted between the radio base station and the radio terminal. As described above, if the transmission power of the radio terminal calculated by applying the PL of the UL by the radio base station is different from the transmission power of the radio terminal calculated by applying the PL of the DL by the radio terminal, it is highly probable that the UL scheduling performed for the radio terminal by the radio base station is inappropriate. That is, in such situations, communication quality between the radio base station and the radio terminal may tend to deteriorate.
Particularly, when the AF relay node relays radio communication between the radio base station and the radio terminal, the AF relay node performs an amplification process. Due to the amplification process, a delay time occurs in a signal transmitted between the radio base station and the radio terminal in a radio manner. When the delay time is larger than a CP (Cyclic Prefix) length in an apparatus (for example, the radio terminal) at a reception side, the apparatus at the reception side simultaneously receives a direct wave directly reaching from a wave source (for example, the radio base station), and a delayed wave having a large delay time. Therefore, inter-symbol interference, inter-carrier interference, and deterioration of the accuracy of channel estimation occur, resulting in the deterioration of reception quality. Particularly, since the deterioration of the accuracy of the channel estimation in the apparatus at the reception side is largely affected as the order of the modulation and coding scheme is increased, the deterioration of the reception quality is increased. Particularly, when the apparatus at the reception side is a terminal, since the performance of the channel estimation is limited, the terminal is largely affected by the influence due to the deterioration of the accuracy of the channel estimation caused by delay.